Liquid crystal displays may be used advantageously in many applications. For example, liquid crystal displays are extensively used in calculators, digital watches, digital clocks, and very small television displays.
Conventional liquid crystal displays include a plurality of pixels arranged in rows and columns. Each pixel usually includes two electrodes formed on spaced apart substrates with the liquid crystal material located between the electrodes. Depending upon the optical properties of the material, either a transmissive or a reflective display can be formed.
To control a display, selected electrical potentials are applied across the electrodes. The electrodes in turn impress a field onto the liquid crystal material causing it to take a desired orientation, thereby forming the transmissive or reflective display. In order to form a complex display a substantial number of pixels is necessary. Each pixel must be addressable, usually by means of x and y address lines. Further, experience has shown that it is very desirable when using nematic liquid crystal display material to alternate the polarity of the potentials applied to the electrodes for the purpose of extending the life of the display material. To increase the isolation between pixels, isolation devices such as diodes or transistors are often used in connection with each electrode. In view of the large number of isolation devices needed, at least one per electrode, it is desirable to form them on the substrate adjacent the respective display electrode. Exemplary displays of this type are disclosed in U.S. patent application Ser. No. 573,004, entitled "Liquid Crystal Displays Operated by Amorphous Silicon Alloy Diodes," filed Jan. 23, 1984 and assigned to the Assignee of the present invention.
If a display has, for example, 1,000 pixels, at least 1,000 operative isolation devices must be formed on the substrate. It is well known that in the manufacture of integrated or thin film semiconductor devices that a certain percentage of the devices which are formed will be defective for various reasons. Because of the failure rate in forming isolation devices, pixels in a display can be unusable due to inoperative isolation devices. Thus, the ability to test a display quickly to determine if it is usable is important.